1. Field of the Invention
The present invention relates to an imaging lens with high resolution and large aperture ratio, for imaging in broadband light such as white light, and with broad angle of field, and for example, relates to a small-sized imaging lens mounted on portable telephones and the like.
2. Description of the Related Art
Recently, downsizing of the imaging lens is progressing rapidly, accompanying the transition from photograph film to imaging elements such as CCD and CMOS. Since imaging lens is mounted on portable telephones and the like, there are increasing demands for reducing cost by mass consumption, and on the other hand, there are demands to solve technical problems such as extreme reduction in thickness and restrictions in light incident angle of the imaging elements.
Further, along with the downsizing of the imaging element, pixels are increasing, and the pixel size is minimized significantly. The imaging lens to be applied to such imaging elements is not only required to have various aberrations corrected satisfactorily, but also highly requested to have large aperture ratio in order to obtain sufficient diffraction resolution. In order to satisfy such requests, there are demands for a more compact imaging lens, having larger aperture ratio and having high performance with reduced costs.
Accompanying increased pixels of the imaging elements, for example, imaging lens of five-lens configuration are proposed in Japanese Patent Laid-Open 2007-264180 (Patent document 1), Japanese Patent Laid-Open 2010-197665 (Patent document 2), and Japanese Patent Laid-Open 2010-262269 (Patent document 3).
Recently, the superficial content where the imaging element receives light has a tendency of increasing in proportion to increase in pixels of the imaging element. Therefore, even when the aperture ratio of the imaging lens is around F/2.8, it had been possible to provide practical imaging lens such as those shown in Patent document 1 or Patent document 2. However, the downsizing and increasing in pixels of the imaging element is further enhanced recently, and imaging elements having pixel size of approximately 1 μm have been provided. Generally, when the pixel size decreases, the light receiving area of each pixel decreases, so that the image obtained through the imaging element becomes dark. In order to correspond to such small pixel size, fast imaging lens is desired. That is, the imaging lens is required to achieve F/2.4 through F/1.8 and to obtain sufficient diffraction limit. Patent document 3 responds to these requests by providing an imaging lens of five-lens configuration having approximately F/2.4 through F/2.04, however, it cannot be said that sufficient performance is realized. Further, since glass material is heavily used, it is impossible to realize cost reduction.
In order to realize both downsizing and increasing performance, the imaging lens of five-lens configuration are proposed in Patent documents 1 through 3, however, it is difficult to realize increased aperture ratio and enhanced performance. Further, when plastic material is selected for cost reduction, since the applicable lens material is limited, it is difficult to achieve both correction of chromatic aberration and correction of other various aberrations.